The present invention relates to an apparatus for the safe and compact storage of nuclear reactor fuel assemblies.
It is well known to safely and efficiently store nuclear reactor fuel assemblies in a uniform array of discrete shield units by supporting the units in a rigid structure that maintains a safe separation distance under normal and seismically perturbed conditions. It is essential that the safe separation distance be maintained between every fuel assembly and each adjacent assembly in order to avoid a critical geometry. The crucial dimensions in fuel storage rack design are the cell pitch, which is the distance between corresponding points in adjacent cells of the array of shields, and the separation distance, which is the distance between adjacent shield units.
The special areas devoted to storing nuclear fuel assemblies within nuclear power plants were designed under the assumption that spent fuel would be stored for only a few years before shipment to reprocessing plants. Recently the need has arisen to store many more fuel assemblies on site. Thus, recent prior art discloses several fuel storage racks designed to decrease the safe cell pitch from over 50 centimeters down to less than 28 centimeters which for typical commercial pressurized water reactor fuel provides storage capacity for more than 12 fuel cycles. For example, U.S. Pat. No. 4,004,154 and U.S. Pat. No. 4,044,267 disclose an apparatus that uses the principle of the neutron flux trap for the safe yet compact storage of the fuel assemblies in discrete stainless steel or borated stainless steel shield units. This inventive concept can be used to minimize the theoretical cell pitch and separation distance (therein called neutron attenuation distance) for a given effective U-235 enrichment of the fuel in the assembly, regardless of the shield unit material.
Since the degree of subcriticality, and hence safety, of an array of stored fuel assemblies is not usually measured in the field, very precise and detailed criticality calculations are used to assure the safety of a particular storage rack design. Such calculations are typified by the curves in FIG. 4 of U.S. Pat. No. 4,044,267 which show minimum safe separation distance as a function of effective enrichment based on calculations made under the assumption that there are no uncertainties in the shield unit dimensions and spacing. The nominal design values of the crucial dimensions of the manufactured racks additionally include the following allowances:
a. geometric changes resulting from seismic disturbances, PA1 b. tolerance on the dimensions of the shield unit, PA1 c. tolerance on the squareness of the rigid structure supporting the shield units, PA1 d. tolerance on the location of the shield unit with respect to the support structure, PA1 e. bowing of the shield between structural support points. Thus the storage capability of a fuel rack design not only depends on how efficiently the shield units can be theoretically configured to minimize the reactivity of the array, but also on the degree to which the design of the supporting structure, the manufacturing tolerances, and the construction tolerances necessitate increasing the theoretical minimum dimensions to obtain nominal dimensions used for manufacturing. Typical prior art structures require allowance for items a - e above of about 1.5 centimeters.
Discrete shielding units are also commercially available in the form of a strong neutron absorber, boral, (which contains boron carbide in aluminum) sandwiched between inner and outer layers of stainless steel or aluminum. The boral is not permanently bonded between the stainless layers but is held in place primarily by means of a plurality of small horizontal ridges. These shields can be used in conjunction with the flux trap principle for minimizing the separation distance between shields. The commercial literature shows a structural support framework for an array of boral shield units consisting of multiple levels of grids into which the units are loosely inserted. Although the safe cell pitch for the boral type shield units is generally smaller than the cell pitch for the boron stainless units, prior art structures for supporting the boral shields also require that significant allowance be added to the theoretical minimum cell pitch in order to determine the nominal dimensions of the rack.